Here is why Google AdWords counts its own sales better than Google Analytics can count those sales -

Google Analytics often mis-credits sales to sources that weren't the ones to drive the customers to the site, such as the store's domain and checkout services, emails and onsite's promotions and the channel "Direct/none".

Store's domain and checkout services should be added to referral exclusion list - see here how.

Emails, onsite's promotions and Direct/none are the reason that Google AdWords Conversion Code will always be more accurate as it would not lose its credits for those channels.